Network analyzers which are used for determining scattering parameters of connected devices under test, such as filters, amplifiers and attenuation elements, cover a broad frequency range and are achieving increasingly upper frequency limits. The scattering parameters of devices under test can be determined, for example, in the W band (75 GHz to 110 GHz), via special upconverter units. In this context, the individual components of a network analyzer themselves are affected by error and provide a frequency response and a phase response, which, especially in the case of high frequencies, prevents accurate measurement of the device under test. Calibrating the network analyzer provides a compensation of these system errors, thereby significantly increasing the measurement accuracy.
Such a calibration is achieved by connecting different calibration standards with known electrical properties to the test ports consecutively and determining the measured values. Known calibration standards are “open” (English: open), “short” (English: short), “match” (English: match) and “through” (English: through). In this context, for example, the calibration standard “match” must achieve the maximum possible signal attenuation over the broadest possible frequency range, so that a clear differentiation is guaranteed, for example, between the calibration standard “match” and the calibration standard “short”. The calibration standards “open”, “short” and “match” are provided in this context as coaxial connectors, which are screwed to the individual coaxial test ports of the network analyzer. To transmit signals with frequencies up to 110 GHz via a coaxial connection, coaxial lines, of which the outer conductor diameter is 1 mm and the inner conductor diameter is 0.434 mm, are used. The calibration standards embodied as coaxial connectors must accordingly be manufactured in a very precise and at the same time mechanically robust manner because even deviations of 0.005 mm in the outer conductor diameter cause the wave impedance to vary by approximately 0.7 ohms, thereby preventing high-precision measurements.
A termination for a coaxial line system is known from U.S. Pat. No. 5,047,737 A. An inner conductor provides a groove, which is used to host a substrate. The substrate is coated with a thin-film resistor, which is connected to the inner conductor by means of a soldered connection. The thin-film resistor connects the inner conductor to the housing. In this context, the borehole within the housing is tapered towards the end. The disadvantage with U.S. Pat. No. 5,047,737 A is that the junction from the inner conductor to the substrate represents a considerable disturbance of the electromagnetic field and therefore reduces the efficiency of the attenuation element.
Accordingly, there is a need to provide a calibration unit which is, on one hand, mechanically robust, and, on the other hand, also achieves optimal properties at very high frequencies, and to provide a corresponding manufacturing method.